Push processing is a technique that is used often in the photographic industry to correct for intentional or unintentional underexposures. In essence, photographers who have underexposed a photographic element--for instance, photographers who exposed a slow film at a faster than appropriate speed at an athletic event in order to photograph a participant or object in rapid motion--can compensate for the relatively small amount of silver that was formed in the underexposure, thus recapturing lost speed, by prolonging the development of the film in the black and white first developer.
Often, however, push processing results in a degradation of color balance as the increase in speed of one color record does not match that of the other color records in the element. It is desirable to provide methods which correct color imbalance independently and selectively (of specific color record and of specific density) during push processing. These methods comprise mechanisms for development inhibition and acceleration during extended first development times. U.S. Pat. No. 5,460,932 describes photographic elements containing development accelerators and compounds that release development inhibitors.
U.S. Pat. No. 5,354,650 describes photographic elements containing compounds that release development inhibitors after extended development times. Consequently these inhibitors impact the characteristics of the photographic element primarily after the initial development phase in the region of the element in which they are located. This allows one to affect color balance by slowing the development of one silver halide emulsion layer during the push phase while simultaneously allowing the other silver halide emulsion layers to continue developing without restraint.
Development accelerators are described in U.S. Pat. Nos. 3,535,487; 5,041,367; 5,460,932; and Research Disclosure December 1989, Item 08119, Sections XXI B-D. Development accelerators can also include such things as competitors for oxidized developer as described in U.S. Pat. No. 4,923,787; fine grain silver halide crystal (e.g. Lippmann) or fine grain silver (e.g. Carey Lea Silver), or surface or internally fogged silver halide grains as exemplified in U.S. Pat. Nos. 4,656,122; 4,082,553; 2,996,382; 3,178,282; 3,397,987; and 4,626,498. These development accelerators when incorporated in silver halide emulsion layers act to shift the emulsion characteristic curve towards lower exposure (faster speed) for each density level, often with greater effect in the lower scale than in the upper scale of the final sensitometric curve. Typically, the development accelerators achieve their effects by affecting silver development or dye formation. Also, typical development accelerators increase fog in the first developer, resulting in a loss of Dmax in the final sensitometric curve for reversal systems upon push processing and, in some cases, even with normal processing times.
The present invention provides push acceleration of the specific emulsion layer (exposure/density region) without degradation of Dmax (by increased first development fog) under both normal and push processing conditions. The present inventors have discovered that adding certain benzazolium compounds to specific layers of a multilayer film element, particularly a reversal film element, can increase the speed of a specific portion of a color record upon push processing when such an increase is desired.
Benzazolium salts have been utilized for various purposes in photographic elements. U.S. Pat. No. 3,326,681 describes the use of benzothiazolium salts containing carboxyalkyl or sulfoalkyl substituents as being useful in a diffusion transfer system as antifoggants. U.S. Pat. No. 4,578,348 describes the speed/fog relationships of certain benzothiazolium salts. U.S. Pat. No. 5,149,619 describes the use of methoxy benzothiazolium salts in silver chlorobromide infrared sensitive emulsions to increase speed. U.S. Pat. No. 5,320,938 describes the use of benzothiazolium salts as incubation stabilizing agents when added after sensitization of high chloride tabular grain emulsions. However, none of these references describes or suggests the use of a certain class of benzazolium salts to improve color balance in reversal elements when "push processing" is utilized.